Calculation of Drag Coefficient, Help Please
 

I am currently designing a solar car for my final year project at university. The main property I am interested in for this simulation is finding the drag force. I have successfully managed to run these simulations and find the drag force.

However I am required to verify the accuracy of these results. I decided to verify the accuracy of these results by simulating a sphere in air flow and using the drag force given by CFX to find the drag coefficient however when I do this the drag coefficient I find is approximately one third of the real value!!!

Additionally I then decided to verify the results used the Ahmed model and again, the drag coefficient calculated is one thirs of the real value!

I have double checeked all my calculations and I am sure they are correct. So I think the problem lies in the model itself.

What are the essential requirements needed in the model and mesh in order to find an accurate drag force? Any Help would be appreciated.

Additionally when I compare the simulated streamlines to those found experimentally the streamlines match exactly what is expected for the ahmed model. So I am unsure as to why my values for drag force are off.

Edit: I am using the SST model, with a RMS of 1e-5. Is the SST model the best option?

I also read somewhere that for an accurate drag calculation the required yplus value is less than 1! Should this be adhered to strictly in order to get an accurate result?